The present invention relates to a contour emphasizing circuit, wherein contour components in the horizontal and vertical directions are detected from an input video signal (e.g., digital input video signal), the detected contour components in the two directions are added, the resulting sum is multiplied by a coefficient, and the resulting product is added to an incoming video signal (e.g., incoming digital video signal) to obtain a contour-emphasized video signal.
As thin and lightweight display devices, the PDP (Plasma Display Panel) and LCD (Liquid Crystal Display) are becoming popular among users. Each of these display devices is directly driven by digital video signals and comprises a contour emphasizing circuit, such as the one shown in FIG. 1, to obtain a contour-emphasized image based on the contour-emphasized video signal obtained from an incoming video signal.
The contour emphasizing circuit shown in FIG. 1 comprises a vertical contour component detection stage 10, a horizontal contour component detection stage 12, a first adder 14, a second adder 16 and a multiplier 18. The vertical contour component detection stage 10 comprises 1-line delay elements 22 and 24 (e.g., line memory), an adder 26 for adding the output signal from the 1-line delay element 24 and the video signal Y input to input terminal 20, a multiplier 28 for multiplying for output the sum obtained by the adder 26 by a coefficient xc2xc, a multiplier 30 for multiplying for output the signal output from the 1-line delay element 22 by a coefficient xc2xd, a subtracter 32 for subtracting the output signal of the multiplier 28 from the output signal of the multiplier 30 and a 1-dot delay element 34 for outputting the signal from the subtracter 32 after giving a time lag for 1 dot, wherein the contour component in the vertical direction VE (hereinafter referred to as vertical contour component) is detected, for output, from a picture element immediately above and a picture element immediately below a reference picture element.
The horizontal contour component detection stage 12 comprises 1-dot delay elements 36 and 38, each sequentially delaying for 1 dot the output signals from the 1-line delay element 22, an adder 40 for adding the output signal of the 1-line delay element 22 and the output signal of the 1-dot delay element 38, a multiplier 42 for multiplying for output the sum obtained by the adder 40 by a coefficient xc2xc, a multiplier 44 for multiplying for output the output signal of the 1-dot delay element 36 by a coefficient xc2xd and a subtracter 46 for subtracting the output signal of the multiplier 42 from the output signal of the multiplier 44, wherein the contour component in the horizontal direction of the reference picture element HE (hereinafter referred to as horizontal contour component) is detected, for output, from the picture element immediately on the left and a picture element immediately on the right of the reference picture element.
The first adder 14 adds the vertical contour component VE detected by the vertical contour component detection stage 10 to the horizontal contour component HE detected by the horizontal contour component detection stage 12, the multiplier 18 multiplies, for output, the sum (VE+HE) obtained by the first adder 14 by a variable coefficient K, and the second adder 16 adds the output signal of the multiplier 18 to the output signal from the 1-dot delay element 36 (input video signal Y having a time lag for 1 line and a time lag for 1 dot) and outputs a contour-emphasized video signal (Y+Kxc3x97(VE+HE)) to the output terminal 48. The contour emphasizing rate varies with the variable coefficient K.
However, the contour-emphasizing circuit shown in FIG. 1 has a problem that the multiplier 18 multiplies the output signal from the first adder 14 by a common coefficient K regardless of whether the contour component (VE or HE) is output from one of the vertical contour component detection stage 10 and the horizontal contour component detection stage 12 or whether the contour components (VE and HE) are output from both, thereby causing the contour including both the vertical contour component and the horizontal contour component to be emphasized excessively to end up with an image including an unnatural portion.
In other words, as shown in FIG. 2, in a case such as when an image of any object is displayed in a background image, there exists a vertical contour component detection point (picture element denoted by ∘ in the diagram) where the vertical contour component VE is detected, horizontal contour component detection point (picture element denoted by xcex94 in the diagram) where the horizontal contour component HE is detected and vertical-and-horizontal contour component detection point (picture element denoted by ▪ in the diagram) where both the vertical contour component VE and horizontal contour component HE are detected.
The contour component at the vertical-and-horizontal contour component detection point (picture element denoted by ▪ in the diagram) is equivalent to the sum of the vertical contour component VE and horizontal contour component HE (VE+HE), thereby causing a problem that the contour representing this detection point is excessively emphasized creating unnatural portion of the image. In FIG. 2, the block demarcated with the dotted line corresponds to one picture element.
The present invention is devised in consideration of the above problems of the prior art, and the object thereof is to prevent the excessive emphasis on the contour at the vertical-and-horizontal contour component detection point where both the vertical contour component VE and horizontal contour component HE are detected. In other words, the object of the present invention is to enable natural emphasis of the contour by preventing the contour at the vertical-and-horizontal contour component detection point from being overemphasized relative to other portions of contour (e.g., contours represented by the vertical contour component detection point and the horizontal contour component detection point).
The contour emphasizing circuit according to the present invention is designed so that the horizontal contour component HE and vertical contour component VE are detected from an input video signal, the detected horizontal contour component HE and vertical contour component VE are added, the sum is multiplied by a coefficient, and the product is added to the input video signal to generate a contour-emphasized video signal, while the contour emphasizing circuit comprises a comparison/determination stage for comparing the detected horizontal contour component with the detected vertical contour component to determine whether or not the absolute value of the difference between them is less than preset value S and a coefficient-selective multiplication stage for multiplying the sum of the horizontal contour component HE and the vertical contour component VE by selectively applying a coefficient K1 or K2 (0 less than K1 less than K2).
The preset value S of the comparison/determination stage is preset so that the absolute value of the difference between the horizontal contour component HE and the vertical contour component VE becomes less than the present value S when both HE and VE are detected, and the absolute value of the difference between the horizontal contour component HE and the vertical contour component VE exceeds the preset value S when one of HE and VE is detected.
Therefore, at the detection point (i.e., picture element, the same applies hereinafter) where only one of the horizontal contour component HE and vertical contour component VE is detected, the absolute value of the difference between HE and VE will not become less than the present value S, so that the coefficient of the comparison/determination stage becomes K2 (e.g., substantially equal value to K of the prior art), and the contour component to be emphasized becomes K2xc3x97HE or K2xc3x97VE, which is substantially equal to Kxc3x97HE or Kxc3x97VE of the prior art.
Further, at the detection point where both the horizontal contour component HE and vertical contour component VE are detected, the absolute value of the difference between HE and VE becomes less than the preset value S, so that the coefficient of the coefficient-selective multiplication stage is changed to smaller K1 from K2 by means of a determination signal (e.g., H-level signal) of the comparison/determination stage, and the contour component to be emphasized becomes K1xc3x97(HE+VE), which is a smaller value (e.g., xc2xd) than Kxc3x97(HE+VE) of the prior art. Thus, it becomes possible to prevent the contour at the point where both HE and VE are detected from being overemphasized, thereby providing a natural emphasizing of the contour.
Further, for easier composition, the coefficient-selective multiplication stage may be made to comprise a first multiplier for multiplying, for output, the sum of the horizontal contour component HE and vertical contour component VE by the coefficient K1, a second multiplier for multiplying, for output, the sum of the horizontal contour component HE and vertical contour component VE by the coefficient K2, and a selector for selecting, for output, either the output signal from the first multiplier or the output signal from the second multiplier.
Further, the coefficient K1 of the coefficient-selective multiplication stage may be set to xc2xd of the coefficient K2 so that the emphasis on the contour at the point where both the horizontal contour component HE and vertical contour component VE are detected is made substantially equal to the emphasis on the contour at the point where only one of the horizontal contour component HE and vertical contour component VE is detected, thereby enabling an extremely natural emphasis on the contour.